When a body of aluminum is melted in a furnace for purposes of casting or the like, dross forms on the surface of the molten aluminum and must be periodically removed, for example by skimming or similar operation. The removed dross generally contains economically substantial amounts of free aluminum as well as aluminum oxides and certain other metals and metal salts, such as magnesium, manganese, and lithium depending on the nature of the aluminum or aluminum alloy being treated.
U.S Pat. No. 4,877,448 discloses a process for the recovery of free aluminum and aluminum oxides from aluminum dross and aluminum scrap (hereinafter, for convenience, collectively referred to as "dross" or "aluminum dross") comprising heating the dross in a high temperature furnace using a plasma arc torch fed with air, or other oxidizing gas such as a mixture of air and oxygen or air and nitrogen as the arc gas. The use of air as the arc gas, in comparison to nitrogen
(1) provides 40% more heat output with the same electrical input (KW/hr), resulting in a shorter processing time and substantially higher throughputs;
(2) provides reduced nitride formation since the hottest part of plasma which is closest to the torch is oxidizing, not nitriding;
(3) provides an oxide, Al.sub.2 O.sub.3, which is stable and not an environmental problem in contrast to a nitride which is produced with nitrogen which is unstable and is an environmental problem; and
(4) is more economical since air is much cheaper than nitrogen or argon, or any other available inert arc gas.
The ability to use air as the arc gas and the enhanced results obtained was surprising. It was to be expected that the preferred gas would be nitrogen or other non-oxidizing gas such as argon, neon, and the like so as to avoid oxidation and burning of the aluminum as would be expected to occur in the presence of air. However, it was found that when using nitrogen as the arc gas nitriding occurred, causing difficulty in the operation of the torch and in substantial loss of aluminum; whereas air not only did not burn to any substantial degree, but provided highly beneficial results.
Excessive oxidation of the aluminum does not occur when air is used as the arc gas because of the very limited amount of air admitted through the plasma torch; and, accordingly, the aluminum burning is controlled. The loss of aluminum is lower than that lost due to nitriding with nitrogen as the arc gas, and additionally increased heat output with the same electrical KW/hr input is obtained. This, in turn, results in the shorter processing time and substantially higher, approximately twice, dross throughput.
Plasma torches, including torches used in processing dross using air as the arc gas, have been conventionally operated at about 3,500-4,000 BTU/#arc gas. Higher enthalpy has been avoided in that higher enthalpy has been believed to lead to higher electrode consumption. Moreover, at higher enthalpy heat loss within the torch is greater, thus leading to inefficiency.